Oxidation of Superalloys in Extreme Environments

Abstract

With the current interest in increased efficiency and reduced emissions, materials in power generation systems are increasingly exposed to more aggressive environments and higher temperatures. It is important to understand how these environments can increase corrosion rates, possibly reducing performance and likely shortening component lifetimes. Superalloys like 718 and its derivatives primarily rely on the formation of an external Cr-rich oxide layer or scale for environmental protection at typical application temperatures (≤700°C). While many laboratory oxidation and creep experiments are conducted in ambient air, the application environment for these alloys often involves combustion products, such as H 2 O and CO-CO 2 . The effect of these environments is not completely understood, especially the impact of long-term exposures, typically required of superalloy components in turbines. Air and fuel impurities, especially S and Na, can result in accelerated degradation such as the well known hot corrosion attack. While the effect of environment on crack growth has been studied, less information is available on the effect of environment on creep and fatigue properties. Initial work is being conducted to better understand the role of environment on creep properties, eventually including in-situ testing. Examples are given from steam and wet air environments and comparisons of different superalloy compositions with cast versus wrought microstructures.

Effect of Environment on the High Temperature Oxidation Behavior of 718 and 718Plus

“…The 718Plus specimen exhibited a thinner reaction product after 5,000 h in wet air at 700°C with less internal oxidation, Figure 3h. Previously, similar results were shown with and without water vapor after 1,000 h exposures including increased Cr depletion with water vapor [22,23]. The Cr Figure 4 shows similar EPMA composition profiles for 718Plus after 5,000h at 650°C in wet air.…”

“…For the more relevant wet air environment, the presence of O2 and H2O results in the formation of a volatile oxy-hydroxide [18][19][20] resulting in a net mass loss at both temperatures. Mass losses for Ni-base alloys exposed to H2O-containing environments at high temperature have been observed previously [21][22][23]. Figure 3 shows cross-sections of each of the alloy coupons after 5,000 h exposures (except the 10,000h exposure shown in Figure 3a) and provides direct comparison of the scales formed on 718 and 718Plus in most of the environments shown in Figure 2.…”

“…Mass change data in air and steam can be used to generate rate constants, however, in the presence of O2 and H2O, a volatile CrO2(OH)2 reaction product forms [17][18][19][20][21][22][23]. The net mass loss makes quantification of this linear reaction mechanism more difficult especially with the small Cr loses near application temperatures of 650°-700°C.…”

Recent Advances of High Entropy Alloys: High Entropy Superalloys

“…These amalgams are used for power generation, industrial, marine and aerospace applications [10]. They are characterized by their excellent heat and oxidation resistance at elevated temperatures, high melting temperature, and high-temperature mechanical strength, good fracture toughness, and stress-rupture, creep resistance [18]. In general, superalloys contain more of Co, Ni, Cr or Fe but less of Ta, Hf, W, Cr, B, Mo, Nb, Al, Zr, C, Ti because these elements adversely affect the properties of the blend.…”

Oxidation Behavior of Non-Modified and Rhodium- or Palladium-Modified Aluminide Coatings Deposited on CMSX-4 Superalloy

“…[2]. Exposure of superalloys to an environment of high-pressure turbine engines leads to their degradation from oxidation, hot corrosion and thermal fatigue [3]. Usage of nickel-based superalloys covered by diffusion aluminide coatings ensures high-temperature oxidation resistance of turbine blades and vanes [4].…”